As is known, packages of this sort are produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized in the packaging unit, e.g. by applying a chemical sterilizing agent such as a hydrogen peroxide solution, which is subsequently removed, e.g. by heating and evaporation, from the surfaces of the packaging material.
The sterilized web is maintained in a closed, sterile environment, and is folded into a cylinder and sealed longitudinally to form a tube.
The tube is fed in a vertical direction parallel to its axis, and is filled continuously with the sterilized or sterile-processed food product.
Packaging units are known which interact with the tube to heat seal it at equally spaced cross sections to form pillow packs connected to the tube by transverse sealing strips.
More specifically, the unit comprises two forming assemblies movable along respective runners and which interact cyclically and successively with the tube to heat seal the packaging material of the tube.
Each forming assembly comprises a slide movable back and forth along the respective runner; and two jaws hinged at the bottom to the respective slide, and movable between a closed position cooperating with the tube to heat seal it, and an open position detached from the tube.
The movements of the forming assemblies are offset by a half-period: the slide of one forming assembly moves upwards with its jaws in the open position, as the slide of the other forming assembly moves downwards with its jaws in the closed position to prevent collision and interference.
The jaws of each forming assembly have respective sealing members which cooperate with opposite sides of the tube and comprise, for example, a heating member, and an elastomeric member for providing mechanical support to grip the tube to the required pressure.
Each forming assembly also comprises two forming members comprising respective forming half-shells hinged to the respective jaws.
Each pair of forming half-shells moves cyclically between an open position detached from the tube, and a closed position contacting the tube to fold it between two consecutive sealing sections and so define and control the volume of the package being formed.
More specifically, the sealing members of a first forming assembly seal the package at the bottom, and the half-shells of the first forming assembly control the volume of the package being formed, while the sealing members of the second forming assembly seal the top of the package.
To conform with the legal requirements imposed in certain countries, the volume and/or weight of the food product in the package must be controlled as accurately and repeatably as possible.
A need is felt within the industry to control the volume of the food product in the package without stopping the packaging unit and so reducing the output rate of the unit, and without leaving any marks on the packages themselves.
A need is also felt within the industry to control the above volume for a wide range of food products of widely differing viscosity, and/or for a wide range of output rates of the packaging unit.